The 3rd Generation Partnership Project, 3GPP, is responsible for the standardization of the Universal Mobile Telecommunication System, UMTS, and Long Term Evolution, LTE. The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network, E-UTRAN. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative to UMTS. In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 Hz when carrier aggregation is employed. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex, FDD and Time Division Duplex, TDD, modes.
Device-to-device communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite such as Wi-Fi Direct. These systems operate in unlicensed spectrum.
Recently, device-to-device, D2D, communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Typically, it is suggested that such device-to-device communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes is a less likely alternative as spectrum is a scarce resource and dynamic sharing between the device-to-device services and cellular services is more flexible and provides higher spectrum efficiency.
Devices that want to communicate, or even just discover each other, typically need to transmit various forms of control signaling. One example of such control signaling is the so-called discovery signal, also referred to as beacon signal or discovery beacon signal, which at least carries some form of identity, referred to as a D2D ID in this disclosure. The discovery signal may possibly carry additional information that is useful for the discovery service and is transmitted by a device that wants to be discoverable by other devices.
Other devices may scan for the discovery signal. Once they have detected the discovery signal, they can take the appropriate action, for example to try to initiate a connection setup with the device transmitting the discovery signal.
A reference discovery payload of 104 bits of discovery information plus 24 bits of CRC may be considered, as an indicative value, according to simulation assumptions in RANI. From internal assessments, it results that the D2D ID might be in the order of 80 bits in length. Standardization of D2D is ongoing in 3GPP and more refined numbers are not available at the moment of writing this disclosure. In this disclosure, M refers to the total number of payload bits in a discovery beacon. From the perspective of this disclosure, it is not essential if M includes the CRC bits or not, if any.
A user equipment, UE, here also referred to as a wireless terminal participating in discovery transmits discovery information that is potentially unique in its discovery signal. A UE that is trying to discover the first UE will try to extract the discovery information from the received discovery signal. If the second UE is successful it will call the first UE as discovered.
ProSe (Proximity Services; see 3GPP feasibility study TR 22.803) defines two types of discovery: open and restricted. With open discovery, at least at the first discovery occasion, the discovery information of a UE is not known at receiver in advance. In this case receiving discovery information is simple decoding.
In case of restricted discovery, the receiver attempts detection of a certain specific discovery signal. And the discovery information of the transmitting UE is known at the receiver before attempting discovery. According to a recent proposal (3GPP contribution paper R1-134627), the receiver does not need to successfully decode the whole discovery information, at least for restricted discovery. Instead, it may do what is referred to as “partial bit matching,” which implies that only some of the bits may be correctly decoded by the receiver. An empirical bit error rate (BER) is calculated. If the BER does not exceed a certain threshold, then the receiver determines that discovery information is successfully extracted and the UE is considered as discovered.
Partial bit matching can lead to some false detection. A reason for false detection is that some bits may be erroneously decoded which could lead to an incorrect assumption of a match with the set of N compared bits. R1-134627 suggests that the false detection probability may be controlled to some extent by appropriately setting the number of bits to be correctly matched. On the other hand, increasing N reduces the computational efficiency of partial bit matching. Beyond the reduced computational complexity associated to correct decoding/detection of N bits instead of M, R1-134627 states that partial bit matching may provide increased detection probability at a given SNR operating point as compared to the case of full detection. The detection probability increases with a higher BER threshold, however this comes at the cost of increased false detection probability.